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doi:10.2204/iodp.pr.316.2008

Preliminary scientific assessment

Expedition 316 successfully achieved the drilling objectives at the shallow megasplay fault and the frontal thrust. Site C0004, located seaward of the originally proposed site, was successfully cored during Expedition 316. Recovery conditions were difficult because the consolidation state and fractured nature of the prism was not ideal for either extended shoe coring system (ESCS) or rotary core barrel (RCB) coring. Recovery through the fault zone was increased by taking 4.5 m (rather than the typical 9.5 m) cores. This method allowed successful recovery of fault zone samples, enabling description of the deformation surrounding the splay fault. In addition, samples were collected for multidisclipinary postcruise analysis. Whole-round cores were taken for postcruise geotechnical and hydrogeological studies; where these samples were unavailable because of brecciation, split core samples were collected. Testing of samples from beneath the splay fault will evaluate the consolidation state, mechanical properties, and hydrologic properties of the underthrust material. Results from Site C0004 underthrust sediments will be compared to data from correlated material at Site C0008. In addition, Expedition 316 results from Site C0004 will be integrated with LWD data (Expedition 314), and results from Kumano Basin drilling (Expedition 315) will also provide critical information on the timing of splay fault initiation.

The primary unachieved goal at the megasplay region was installment of casing for a future observatory. The casing was originally planned for Site C0003; during Expedition 314, the splay fault could not be reached at this site because of drilling conditions. Instead, Site C0004, which penetrated the splay fault at an estimated depth of ~290 m CSF, was selected as a replacement. The casing plan was not transferred to Site C0004 because of uncertainty as to whether its shallower depth would meet the observatory objectives. In addition, it was felt that if time allowed after completion of Sites C0004 and C0006, it would be scientifically more valuable to attempt a second, deeper crossing of the megasplay fault at ~480 m CSF (which later turned out to be impossible because of time constraints) rather than to begin casing operations.

Drilling of the frontal thrust area provided the material to meet the Expedition 316 objectives in this area. Although Site C0006 did not reach the frontal thrust, it successfully captured several fault zones and characterized the lithologies, ages, deformation, pore fluid geochemistry, and physical properties of the hanging wall. Site C0007 provided a clear view of the high-energy nature of the axial channel environment through recovery of very coarse grained deposits, including gravel. These coarse-grained sediments provide clues to the nature of material accreted into the prism. After moving to Hole C0007C, ~185 m from Hole C0007A, to avoid a fault-thickened section of gravels, the frontal thrust was crossed. Through age reversals and structural observations, it is clear that a significant fault within the frontal thrust was recovered at Site C0007 and that the recovered material documents shear localization.

X-ray CT scan imaging proved to be extremely scientifically valuable during the cruise. It was used to ensure that critical intervals were not removed for whole-round samples for interstitial water or microbiological analyses. It was also crucial for identifying and measuring deformation structures that would otherwise have been masked by the splitting progress and for discerning natural from drilling-induced brecciation. Postcruise analyses of the X-ray CT images will assist in structural analyses and evaluation of slump or slope failure deposits, potentially related to the seismogenic zone.

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